The following abbreviations are herewith defined, at least some of which are referred to within the following description of the prior art and the present invention.
CQIChannel Quality IndicatorsDLDownlinkFDDFrequency Division MultiplexingMIMOMultiple Input Multiple OutputOFDMOrthogonal Frequency Division MultiplexingIIDIndependent and Identically DistributedFFTFast Fourier TransformIFFTInverse Fast Fourier TransformUEUser EquipmentULUplink
The use of multiple antennas at the transmitter and/or the receiver in wireless communication systems has attracted substantial attention over the past decade because of the potential improvement in both coverage and data rate. Unlike single antenna systems where exploiting the channel knowledge at the transmitter does not significantly improve the capacity, the pioneering works of Telatar and Foschini have shown that substantial gain in the capacity can be achieved with multiple antennas when accurate channel state information is available at the transmitter (see reference nos. 1-3). In a frequency-division multiplexing (FDD) system, the provision of such information at the transmitter relies mainly on the use of feedback. While assuming perfect channel state information at the transmitter is unrealistic due to the capacity limitation on the feedback link and its round-trip delay, it has been shown that even partial channel knowledge at the transmitter can provide significant gain when compared to systems without channel information at the transmitter. This has spurred significant interest in designing effective methods of reducing the amount of feedback of channel state information without significantly penalizing the capacity.
An effective approach to reducing the amount of feedback of channel state information without excessively sacrificing accuracy involves exploiting the statistics of the channel. For instance, in the co-assigned U.S. Patent Application No. 2009/0016425 A1, an effective method of compressing the feedback of the instantaneous channel response of a spatially correlated MIMO channel has been described (see reference no. 4). FIG. 1 (PRIOR ART) is a system diagram illustrating a transmitter 102 and a receiver 104 that communicate with one another using a MIMO channel 106 and implement this feedback method which utilizes fast and slow feedback links 108 and 110. The basic idea of this feedback method is to use the knowledge of certain second-order channel statistics at the receiver 104 to compress the channel response information 112. Then, the compressed feedback of the instantaneous channel response 112 is fed back from the receiver 104 to the transmitter 102 using the fast feedback link 108. On the other hand, the channel statistics φTX 114 is provided from the receiver 104 to the transmitter 102 through the low-rate slow feedback link 110 which sends back information much less frequently when compared to the fast feedback link 108.
An important aspect of the feedback method described in U.S. Patent Application Ser. No. 2009/0016425 A 1 is that the receiver 104 applies a two-dimensional linear transformation (across frequency and space) to the samples of the frequency-domain response Hf[k] of the MIMO channel 106 (it is assumed that the receiver 104 is able to obtain accurate estimates of the nR×nT channel matrix Hf[k] for each k th subcarrier). This transformation is used to transform Hf[k] into a vector of transform coefficients X in order to achieve substantial compression benefits. At the receiver 104, the frequency-domain channel response Hf[k] is first converted into a time-domain channel response {H1[n]}n=1N through an inverse fast Fourier Transform (IFFT) operation. According to the assumed maximum delay spread of the system, the time-domain response is then truncated to fewer number of taps within a window of time indices, denoted by W⊂{1, 2, . . . , N}. Each tap of the resulting channel response {H1[n]}nεW is further transformed spatially to obtain a set of transformed vector channel taps {X[n]}nεW, which is then further reduced into a smaller number of parameters before they are quantized into bits and fed back on the fast feedback link 108 to the transmitter 102.
The spatial transformation is done according toX[n]=vec(H1[n]UT)  (1)for all nεW, where UT denotes the matrix with eigenvectors of the channel correlation matrix φTX given by:
                              Φ          TX                ≡                              E            ⁡                          [                                                1                  N                                ⁢                                                      ∑                                          k                      =                      1                                        N                                    ⁢                                                                                                              H                          f                                                ⁡                                                  [                          k                          ]                                                                    H                                        ⁢                                                                  H                        f                                            ⁡                                              [                        k                        ]                                                                                                        ]                                .                                    (        2        )            At the transmitter 102, an inverse spatial transformation is applied to the transformed coefficients so as to obtain an reproduction of the frequency-domain channel response Hf[k]. It is clear, that the transmitter 102 to perform the inverse spatial transformation will need to use the transmit channel correlation matrix φTX and as a result the transmit channel correlation matrix φTX also needs to be fed back by the receiver 104 to the transmitter 102. The receiver 104 can use the low-rate, slow feedback link 110 to send the transmit channel correlation matrix φTX (to the transmitter 102. However, the amount of this feedback is in the order of O(nT2) and can be rather large. Hence, there has been and is a need to reduce the amount of feedback related to the transmit channel correlation matrix φTX, that has to be sent to the transmitter 102. This need and other needs are addressed by the present invention.